The proposed research is directed toward a better understanding of the organization of DNA sequences within the chromosomes of eukaryotes and how this organization may change during the course of evolution. The virilis group of Drosophila will be the set of organisms studied. Members of this group comprise an exceptionally favorable system since, among other things, their evolutionary relationships are well defined and they are amenable to cytogenetic analysis. The locations in chromosomes and properties of highly repetitive, satellite, DNA sequences will be examined by a number of techniques including in situ and filter hybridization. The arrangement of moderately repetitive and unique DNA sequences will be examined using techniques of hydroxyapatite chromatography and electron microscopy. Homologies among unique DNA sequences from members of the group will be determined by DNA reassociation experiments monitored by hydroxyapatitie chromatography. From these studies, it should be possible to determine the relative rates of divergence of highly repetitive, moderately repetitive, and unique DNA sequences in a closely related set of organisms.